Angiogenesis, the growth of new blood vessels, promotes cancer, neovascular eye disease, psoriasis, and rheumatoid arthritis. Adhesion to provisional ECM proteins such as fibronectin is required for endothelial cell survival, proliferation and motility during new blood vessel growth. Our studies demonstrate that two fibronectin receptors, integrins alpha5beta1alpha5beta1 and alpha4beta1, play critical, but distinct, roles in the regulation of angiogenesis. Antagonists of both integrins potently inhibit angiogenesis and tumor growth. However, these integrins differ in their expression patterns and possible modes of action. Integrin alpha5beta1 is expressed on proliferating vascular endothelium in response to activation of the transcription factor Hox D3, which is induced by pro-angiogenic growth factors. Integrin alpha5beta1 then promotes the migration and survival of proliferating endothelial cells in vitro and in vivo. Our studies demonstrate that integrin a5b1 antagonists activate PKA, which then induces endothelial cell apoptosis and inhibition of angiogenesis in vivo. In contrast, integrin alpha4beta1 is poorly expressed by either quiescent or proliferating endothelium in tissues, but is expressed by circulating endothelial progenitor cells. In fact, antagonists of this integrin inhibit the participation of endothelial progenitor cells in growth factor-induced angiogenesis. The overall goal of this proposal is to explore the different mechanisms by which these two distinct fibronectin receptors regulate angiogenesis. We will test the hypotheses that integrin alpha5beta1 regulates vascular cell adhesion, migration, survival and vessel morphogenesis in vitro and in vivo in a PKA-dependent manner. We will also test the hypothesis that integrin alpha4beta1 regulates adhesion, migration, and survival of endothelial progenitor cells and other vascular cells during angiogenesis. Together these proposed studies will delineate the mechanisms by which fibronectin-binding integrins regulate angiogenesis and may lead to new therapies for cancer and other neovascular diseases.